<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>类选择器</title>
    <style>
    .red {
        color: red;
    }
    .size {
        font-size: 55px;
    }
    .blue {
        color: blue;
    }
    </style>
</head>
<body>
    <p class="red">这是p标签</p>

    <div class="blue size">这是div标签</div>
</body>
</html>